Recently, power quality and custom power have been hot topics because of the widespread use of non-linear electronic equipment and the stricter power quality requirements of sensitive loads. To provide high power quality at the point of common coupling (PCC) of a distribution system, line conditioning, including voltage regulation, reactive power (VAR) compensation, and harmonic filtering can provide important advantages.
Traditionally, a multipulse inverter consisting of several voltage-source inverters connected together through a zigzag-arrangement of transformers is used for VAR compensation. These transformers (1) are the most expensive equipment in the system, (2) produce about 50% of the total losses of the system, (3) occupy a large area of real estate, about 40% of the total system, (4) cause difficulties in control due to direct-current (DC) magnetizing and surge overvoltage problems resulting from saturation of the transformers, and (5) are unreliable.
Correspondingly, PWM inverters with .about.10 kHz of high switching frequency have been used for harmonic compensation and static VAR compensation. However, the high initial and running costs have been hindering its practical use in power distribution systems. In addition, it is difficult for PWM inverter based active filters to comply with electromagnetic interference (EMI) requirements (e.g., IEC and FCC Class A & B EMI regulations).
Therefore, there is a continuing need for an improved power line conditioner that is operable for voltage regulation, reactive power correction, and harmonic filtering. In particular, there is a need for a power line conditioner that does not rely on transformers or PWM active filters.